It is known in the prior art to utilize a storage surface within a disk drive device to form a position transducing arrangement for purposes of positioning the data storage heads. The most common method is to provide a dedicated storage surface whereon a servo pattern is magnetically prerecorded. A dedicated magnetic playback or servo head reads the servo pattern and the signal obtained is demodulated to produce a position signal which is applied to a servo system to control an actuator that positions the heads. The servo system is referred to as a track following servo because it essentially follows a track defined by the pre-recorded servo pattern as it positions the servo head on the basis of the demodulated position signal.
In order to provide for a high data storage density in disk drive type devices, the distance between adjacent data tracks must be made very small. This requires that the total positioning error be correspondingly small, such as tens of microniches or less. Therefore, it becomes necessary to reduce the causes of position errors. Since there are usually a large number of items that contribute to position errors and since some of them cannot be easily reduced, it is desirable to reduce as many errors as possible to the extent possible. Among the causes of positioning error is the servo pattern itself.
Position errors caused by the servo pattern itself can be reduced as shown in U.S. Pat. Nos. 3,534,344 and 3,691,543. In U.S. Pat. No. 3,534,344 position information is conveyed by transitions of opposite polarity at circumferentially spaced positions in adjacent servo tracks. In U.S. Pat. No. 3,691,543 position information is conveyed by transitions of like polarity at circumferentially spaced positions in adjacent tracks.
To improve the signal noise ratio, particularly as servo track widths were narrowed to increase the number of tracks per inch, the prior art employed servo heads essentially twice the width of a servo track. However, the use of the servo heads twice the width of the servo track in combination with transitions of like polarity in adjacent servo tracks as in the forementioned U.S. Pat. No. 3,691,543 presented problems where it was necessary to include in the servo pattern dedicated synchronizing pulses for purposes of synchronizing the demodulation process, and track region identifying information and index information.
As the output signal if the servo head is influenced by magnetic transitions proximate one another, peak shifting and reductions in amplitude of the output signal become more pronounced as the transitions are brought or crowded closer together to improve sampling rates. Where the servo pattern is uniform, as when it consists of only position information, the derived position information at the output of the servo head would not have any periodic amplitude abberations due to unequal pulse crowding effects. However, with the introduction of synchronizing and track region identifying information, the pattern is no longer homogenous and the derived position information suffers periodic amplitude abberations due to the pulse crowding influence of the synchronizing and track region identifying information on adjacent position information. These abberations contribute to position errors.
In accordance with the invention, there are included in a servo pattern having synchronizing and track region identifying information, guard transitions between the sychronizing and repetitive position information and between the repetitive position information and track region identifying information which provides pattern uniformity with respect to the position information in that the pulse crowding effect of the guard transitions on position information is the same as the pulse crowding effect between position information, thereby avoiding periodic amplitude abberations in derived position information.
An object of the invention is to provide, in a servo pattern which includes synchronizing, position and track region identifying information, means for avoiding periodic amplitude abberations in the derived position information.
Another object of the invention is in the provision of guard transitions between position information and synchronizing and track region identification included in a servo pattern.